1. Field
The present disclosure relates generally to analog to digital conversion and, in particular, to digitizing analog signals using optical signals. Still more particularly, the disclosure relates to a method and apparatus for analog to digital conversion using modulated optical signals.
2. Background
Digital signal processing is used to measure and/or analyze analog signals. As a first step in digital signal processing, analog to digital conversion is used to convert analog signals into digital data for processing. For example, a continuous analog signal may be converted to digital data in the form of discrete values or numbers.
The resolution of an analog to digital converter is one determining factor for the accuracy of converting signals from analog to digital data. The resolution is the number of discrete values that can be produced over the range of analog values. Typically, these discrete values are stored in binary form, and the resolution is expressed in bits. The accuracy of converting analog signals into digital data is also determined by sources of noise, nonlinear components of the analog to digital converter, and/or other relevant factors.
The effective resolution of an analog to digital converter is limited by the maximum signal-to-noise ratio that can be achieved for the digital signal produced. Thus, an analog to digital converter may only resolve an analog signal to an effective number of bits of resolution (ENOB). The signal-to-noise ratio may depend on a sampling error that may, in turn, depend on the timing jitter. The timing jitter is the time variation in the time period.
Some currently available analog to digital converters use electronic sampling methods for analog signals. These electronic sampling methods have a timing jitter that is greater than about 500 femtoseconds. This timing jitter sets a limit to the signal-to-noise ratio. Other currently available analog to digital converters use optical sampling methods for analog signals. Optical sampling methods can have a reduced timing jitter of less than about 50 femtoseconds. With this reduced timing jitter, optical sampling methods can increase the signal-to-noise ratio as compared to electrical sampling methods.
With currently available systems for analog to digital conversion, the conversion of ultra-wideband input signals into digital signals has a limited effective number of bits of resolution for the digital signals produced. Ultra-wideband is a technology for transmitting information in the form of signals over a bandwidth greater than about 500 megahertz.
Therefore, it would be advantageous to have a method and apparatus that takes into account at least some of the issues discussed above, and possibly other issues.